Abstract: The age of Late Paleozoic volcanic-sedimentary strata in the northwestern Lesser Xingan Range is the key to the regional tectonic evolution. In this paper, we have a research for the zircon U-Pb chronology of Late Paleozoic volcanics and siltstone by using laser ablation ICP-MS technique. The research different from the original shows that the volcanics of Late Carboniferous-Early Permian Baoligaomiao Group, Early Permian Dashizhai Group and Late Permian Wudaoling Group were all formed in Carboniferous, and the zircon U-Pb age includes two periods: 353~352Ma and 307~306Ma. Detrital zircons in the siltstone from the upper part of Honghutuhe Group is of two group ages, and the weighted average 206Pb/238U ages are 385.2±6Ma and 353.0±3.6Ma. The former is similar to the formation age of regional Late Devonian calc-alkaline volcanics and the later is consistent with the age of Early Carboniferous volcanics, which indicates that the Middle Devonian-Early Carboniferous volcanics was the marital source of Honghutuhe Group and the upper limit of sedimentary, are at the Tournaisian of Early Carboniferous. Based on new chronological data and previous stratum finding, we believe that the Early Carboniferous volcanic activity (353~352Ma) accompanied by marine sediments (Honghutuhe Group). However, Late Carboniferous volcanic activity (307~306Ma) associated with continental sediments (Baoligaomiao Group). Combined with the general shortage of Bashkirian in the northern Great Xingan Range, we consider that the stratigraphic transformation from the Early Carboniferous marine sediment to the Late Carboniferous continental sediment is probably related to the collision and merging of the Songnen and Ergun-Xing’an blocks. Therefore, the Early Carboniferous volcanics were probably formed in a pre-collision subduction environment and the Late Carboniferous volcanics were formed in a post-collision extensional setting.